Concentration measurement using luminescence

ABSTRACT

A method of measuring the concentration of a substance, using a sensor ( 2 ) comprising a luminescent material, an LED light source ( 14 ) which supplies a pulse of light to the sensor to cause luminescence of the material, a detector ( 26 ) for detecting the emitted light and a data processing module ( 28 ) for analysing the signals and indicating the concentration of the substance. In a measuring sequence there is a series of pulses of light spaced apart sufficiently to permit the luminescence to decay between pulses. The signals which are analysed are those generated after each pulse of light has terminated and the luminescence is decaying. The data processing module ( 28 ) comprises a plurality of accumulators ( 52 A,  52 B,  52 C and  52 D) which are controlled to be operable over a corresponding plurality of different periods of time during decay of the luminescence. Each accumulator accumulates values indicative of the intensity of the luminescence from the series of pulses in the measuring sequence. The parameters regarding the accumulators are stored in a configuration memory ( 42 ) which is part of the sensor.

This invention relates to the measurement of the concentration of asubstance using luminescence.

It is known to measure the concentration of a substance in livingtissue, such as oxygen, using a probe which includes a luminescentcomponent which is exposed to the substance. Light is directed to thecomponent and the luminescent behaviour is analysed. That behaviourdepends on the concentration of the substance, such as oxygen, whichaffects the intensity and duration of the luminescence. In the paper“Measurement of oxygen tension in tumours by time resolvedfluorescence”, By W. K. Young, B. Vojnovic and P. Wardman, BritishJournal of Cancer (1996) 74, (Suppl. XXVII) S256-S259, there isdisclosed an arrangement in which a fluorophor, namelytris(4,7-diphenyl-1,10-phenanthroline) ruthenium chloride wasincorporated in a polymer which was coated on the end of an opticalprobe. Pulses of light were supplied along an optical fibre from a laserand the exponential decay of the luminescence produced was analysed andused to determine the oxygen concentration.

In U.S. Pat. No. 6,531,097, with inventors B. Vojnovic, W. K. Young andP. Wardman, an alternative system is proposed. A light emitting diode(LED) is used as the light source and instead of analysing the decay ofthe luminescence, the system relies upon analysis of the increase inluminescence. This analysis takes place simultaneously with light beingsupplied. The analysis of the luminescence was carried out by the use ofthree or more integrating circuits, each producing an outputrepresentative of the input signal integrated over time. The lightsource is said to operate for a period of time which is substantiallylonger than the time during which the light emitted by the luminophorchanges following the start of operation of the light source. Whilst thearrangement described is said to have the advantage that it is possibleto use an LED rather than a laser, there are also disadvantages.

In U.S. Pat. No. 6,531,097, the measurement of the increase inluminescence intensity, simultaneously with operation of the excitationlight source, has negative effects on the following:—signal to noiseratio; the stability of the intensity of the light source; andelectro-magnetic noise created by switching the light source. Therequirement that the light source operates for a time substantiallylonger than the time during which the luminescence changes, has anegative impact on the dynamic range of the detected signal and on thephoto-bleaching of luminophores that have long lifetimes.

Viewed from a first aspect, the present invention provides a method ofmeasuring the concentration of a substance, using a sensor comprising aluminescent material having a luminescent activity whose characteristicsdepend on the concentration of the substance, a light source whichsupplies a pulse of light to the sensor for a period of time sufficientto cause luminescence of the luminescent material, the luminescencerising to a peak and then decaying after the end of the pulse of light,a detector for detecting the luminescence and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein there is a measuring sequence comprised of a series of pulses oflight spaced apart sufficiently to permit the luminescence to decaybetween the pulses, the signals which are analysed to provide the dataindicative of the concentration of the substance are those generatedafter each pulse of light has terminated and the luminescence isdecaying, and wherein the data processing module comprises a pluralityof accumulators which are controlled to be operable over a correspondingplurality of different periods of time during the decay of theluminescence, each accumulator accumulating values indicative of theintensity of the luminescence from the series of pulses in the measuringsequence.

Thus, by contrast to the system of U.S. Pat. No. 6,531,097, theluminescence characteristic analysed is the decay of luminescence afterthe light pulse has terminated, rather than the rise in luminescencewhilst the light pulse is operative.

As such, analysing the decay characteristics is known from the paper byYoung et al. but the present invention uses an accumulation technique toprovide the required data. This provides improved results and the methodcan be put into effect with an LED as the light source rather than alaser. Although each pulse from an LED results in a relatively smalloutput of luminescence, there is a series of pulses and the outputs areaccumulated. In some embodiments of the invention it is possible toeliminate readings when the accumulator may still retain a value from aprevious pulse, or when the accumulator reaches saturation.

The periods of time over which the respective accumulators are activeduring the decay of luminescence after a pulse of light, are disposed atchosen points during the decay. The chosen points may depend on thesubstance whose concentration is being determined, and/or the range ofconcentrations that are expected. The rate of decay is steep initially,and then flattens out. Preferably there are at least two accumulatorswhich are timed to be active in the steep part of the curve, andpreferably these are closely adjacent to each other, with a firstaccumulator being active over a first period of time and a secondaccumulator being active over an adjacent period of time which commenceson, or a relatively short time after, the end of the first period oftime. Preferably, there is a third accumulator which is active over athird period of time, which commences a relatively long time after theend of the second period of time and thus when there is substantiallysmaller rate of decay of the luminescence than during the first andsecond periods of time. Preferably, there is a fourth accumulator whichis active over a fourth period of time, which commences a relativelylong time after the end of the third period of time and thus also whenthere is substantially smaller rate of decay of the luminescence thanduring the first and second periods of time.

The periods over which the respective accumulators are active may bedetermined by parameters stored in a configuration memory. Preferablythere is a plurality of different sets of parameters, so that theappropriate set can be chosen having regard to the substance whoseconcentration is being determined, and/or the range of concentrationsthat are expected. In one arrangement, the configuration parameters arestored in a control unit to which the sensor is connected, the controlunit including the data processing module. In this case, the appropriateset of parameters can be chosen manually. Alternatively, the sensor fora particular application may include an identifier such as an RFID chip,so that the control unit can detect the type of sensor and select theappropriate set of parameters.

Preferably, however, the configuration parameters are stored in aconfiguration memory which is part of the sensor itself, such as nonvolatile memory associated with the sensor. When the sensor is connectedto the control unit, a connection is made with the configuration memory,and the configuration parameters are read and used by the dataprocessing module. The connection could be a wireless connection. Theconfiguration memory which forms part of the sensor may store aplurality of sets of configuration parameters appropriate for thatsensor, which may be chosen as appropriate by the control unit.

A configuration memory which forms part of the sensor may store avariety of information, such as any or all of:

-   -   sensor identification information    -   sensor configuration information;    -   the units of measurement for the assay substance;    -   the energy or time exposure of the sensor;    -   the allowed amount of energy or time exposure;    -   the date of calibration of the sensor;    -   the allowed period of time from the date of calibration of the        sensor;    -   the date of first use of the sensor;    -   the allowed period of time from the date of first use of the        sensor;    -   the number of times that the sensor has been used;    -   the allowed number of times that the sensor may be used;    -   light pulsing and accumulator timings;    -   minimum and maximum acceptable luminescent lifetimes;    -   minimum and maximum acceptable sensor temperatures;    -   minimum and maximum acceptable assay concentrations; and    -   sensor dye and temperature calibration information.

Recording the allowed and actual energy or time exposure of the sensorallows the state of photo-bleaching to be monitored and the sensor to beflagged as due for replacement when required. In general there may bestored the value of a parameter relating to the life of the sensor, andthe permitted maximum value of that parameter, for example in additionto or instead of the energy and/or time exposure and the permittedexposure, the date of calibration of the sensor and the allowed periodof time from the date of calibration of the sensor; the date of firstuse of the sensor and the allowed period of time from the date of firstuse of the sensor; and the number of times that the sensor has been usedand the allowed number of times that the sensor may be used.

A sensor which incorporates configuration memory may also incorporate atemperature measuring device, such as a thermocouple with a band-gaptemperature sensor for cold junction correction. In such a case theconfiguration memory may also store any of the following data:

-   -   Minimum and maximum un-calibrated cold junction temperatures;    -   Minimum and maximum calibrated cold junction temperatures;    -   Cold junction calibration information;    -   Minimum and maximum thermocouple voltages;    -   Minimum and maximum calibrated thermocouple temperatures;    -   Thermocouple calibration information.

The use of a sensor with its own configuration memory allows the controlunit to be configured for a wide range of different situations. Forexample, sensors may be optimised for different measurement ranges,luminescent lifetimes and even different assay substances, all measuredby the same apparatus.

In some embodiments of the invention, the periods of time for which theaccumulators are operable are separated from each other by intervals inwhich none of the accumulators is operable. The interval may if desiredbe longer than the respective period of time for which any accumulatoris operable. In some embodiments of the invention, each period for whichan accumulator is operable is spaced from the next period. However, insome embodiments there may be at least some periods which are adjacentwith no interval or overlap. This may be the case at the start of thedecay of luminescence where there is a sharp reduction in output and theperiods may need to be close together. Whilst in some embodiments itcould be the case that there are no intervals between any periods in asequence, in some embodiments there will be an interval between at leastone pair of consecutive periods. There may be at least two, or at leastthree, pairs of consecutive periods for which there are intervalsbetween the periods of the pair.

Where there is an interval between two periods in which an accumulatoris operable, the interval may greater than the duration either of theperiods themselves.

Where there are intervals between periods for which respectiveaccumulators are active, in some embodiments the interval is smallest(or non-existent) for a pair of periods nearest the peak of luminescenceand greatest for a pair of periods furthest from the peak. In someembodiments of the invention, the interval between a pair of periods forwhich respective accumulators are active increases incrementally from apair nearest the peak to a pair furthest from the peak.

Arrangements in which there is an interval between one period and thenext period, differ from the system in U.S. Pat. No. 6,531,097 where theindividual accumulators operate over adjacent periods of time. Thus, inthe described embodiment of U.S. Pat. No. 6,531,097 a first accumulatoroperates over period t0 to t1; a second accumulator operates over periodt1 to t2; and a third accumulator operates over period t2 to t3. Therequirement that the integrating periods are consecutive has a negativeimpact on the noise and accuracy of the results. In embodiments of thepresent invention, accumulation may be carried out over time periodspositioned as required during the decay phase of the luminescence.

In preferred embodiments of the invention, the periods of time overwhich the respective accumulators are operable are of substantiallyequal duration.

In some embodiments of the invention the duration of each period forwhich an accumulator is operable is substantially less than the timebetween the commencement of the first period and the end of the lastperiod. In some embodiments of the invention the duration of each periodfor which an accumulator is operable is substantially less than the timebetween the peak of luminescence after a pulse and the commencement ofthe next pulse. In some embodiments of the invention the duration ofeach period for which an accumulator is operable is substantially lessthan the time for the luminescence to decay after a pulse.

In some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 5% of the timebetween the commencement of the first period and the end of the lastperiod; in some embodiments of the invention, the duration of eachperiod for which an accumulator is operable is no more than about 4% ofthe time between the commencement of the first period and the end of thelast period; in some embodiments of the invention, the duration of eachperiod for which an accumulator is operable is no more than about 3% ofthe time between the commencement of the first period and the end of thelast period; in some embodiments of the invention, the duration of eachperiod for which an accumulator is operable is no more than about 2% ofthe time between the commencement of the first period and the end of thelast period.

In some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 5% of the timebetween the peak of luminescence after a pulse and the commencement ofthe next pulse; in some embodiments of the invention, the duration ofeach period for which an accumulator is operable is no more than about4% of the time between the peak of luminescence after a pulse and thecommencement of the next pulse; in some embodiments of the invention,the duration of each period for which an accumulator is operable is nomore than about 3% of the time between the peak of luminescence after apulse and the commencement of the next pulse; in some embodiments of theinvention, the duration of each period for which an accumulator isoperable is no more than about 2% of the time between the peak ofluminescence after a pulse and the commencement of the next pulse.

In some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 5% of the timefor the luminescence to decay after a pulse; in some embodiments of theinvention, the duration of each period for which an accumulator isoperable is no more than about 4% of the time for the luminescence todecay after a pulse; in some embodiments of the invention, the durationof each period for which an accumulator is operable is no more thanabout 3% of the time for the luminescence to decay after a pulse; insome embodiments of the invention, the duration of each period for whichan accumulator is operable is no more than about 2% of the time for theluminescence to decay after a pulse.

In some embodiments of the invention, the time for the luminescence todecay after a pulse is taken as the time for the luminescence to decayto about 20% of the peak value after a pulse; in some embodiments of theinvention, the time for the luminescence to decay after a pulse is takenas the time for the luminescence to decay to about 15% of the peak valueafter a pulse; in some embodiments of the invention, the time for theluminescence to decay after a pulse is taken as the time for theluminescence to decay to about 10% of the peak value after a pulse; insome embodiments of the invention, the time for the luminescence todecay after a pulse is taken as the time for the luminescence to decayto about 5% of the peak value after a pulse; in some embodiments of theinvention, the time for the luminescence to decay after a pulse is takenas the time for the luminescence to decay to a value less than about 5%of the peak value after a pulse.

In some embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 75 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 80 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 90 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 100 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 120 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 140 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 200 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 250 microseconds; insome embodiments of the invention, the time for the luminescence todecay from the peak after a pulse is at least about 300 microseconds.

In some embodiments of the invention, the time between the peak ofluminescence after a pulse and the commencement of the next pulse is atleast about 75 microseconds; in some embodiments of the invention, thetime between the peak of luminescence after a pulse and the commencementof the next pulse is at least about 80 microseconds; in some embodimentsof the invention, the time between the peak of luminescence after apulse and the commencement of the next pulse is at least about 90microseconds; in some embodiments of the invention, the time the timebetween the peak of luminescence after a pulse and the commencement ofthe next pulse is at least about 100 microseconds; in some embodimentsof the invention, the time between the peak of luminescence after apulse and the commencement of the next pulse is at least about 120microseconds; in some embodiments of the invention, the time between thepeak of luminescence after a pulse and the commencement of the nextpulse is at least about 140 microseconds; in some embodiments of theinvention, the time between the peak of luminescence after a pulse andthe commencement of the next pulse is at least about 200 microseconds;in some embodiments of the invention, the time between the peak ofluminescence after a pulse and the commencement of the next pulse is atleast about 250 microseconds; in some embodiments of the invention, thetime between the peak of luminescence after a pulse and the commencementof the next pulse is at least about 300 microseconds.

In some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 5 microseconds;in some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 4 microseconds;in some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 3 microseconds;in some embodiments of the invention, the duration of each period forwhich an accumulator is operable is no more than about 2 microseconds;in some embodiments of the invention, the duration of each period forwhich an accumulator is operable is between about 1 and 3 microseconds;in some embodiments of the invention, the duration of each period forwhich an accumulator is operable is between about 1.5 and 2.5microseconds; in some embodiments of the invention, the duration of eachperiod for which an accumulator is operable is about 2 microseconds.

In some embodiments of the invention, the time between pulses in theseries is at least about 75 microseconds; in some embodiments of theinvention, the time between pulses in the series is at least about 80microseconds; in some embodiments of the invention, the time betweenpulses in the series is at least about 90 microseconds; in someembodiments of the invention, the time between the time between pulsesin the series is at least about 100 microseconds; in some embodiments ofthe invention, the time between pulses in the series is at least about120 microseconds; in some embodiments of the invention, the time betweenpulses in the series is at least about 140 microseconds; in someembodiments of the invention, the time between pulses in the series isat least about 200 microseconds; in some embodiments of the invention,the time between pulses in the series is at least about 250microseconds; in some embodiments of the invention, the time betweenpulses in the series is at least about 300 microseconds.

In some embodiments of the invention, the duration of each period forwhich an accumulator is operable is relatively short. In someembodiments of the invention, the duration of each period for which anaccumulator is operable can be relatively long, this being particularlythe case if the periods are equal.

In some embodiments of the invention there are at least threeaccumulators; in some embodiments of the invention there are at leastfour accumulators; in some embodiments of the invention there are atleast five accumulators.

In some embodiments of the invention there is an accumulator operablefor a first period shortly after the peak of the luminescence, anaccumulator operable for a second period shortly before the end of thedecay of the luminescence (or shortly before the next light pulse), andat least one accumulator operable for a period arranged between thefirst and second periods. Preferably there is an accumulator operablefor a period arranged between the first and second periods and which isspaced from both of the first and second periods. By shortly after thepeak of the luminescence may mean within no more than about 5microseconds of the peak; or no more than about 4 microseconds of thepeak; or no more than about 3 microseconds of the peak; or no more thanabout 2 microseconds of the peak; or no more than about 1 microsecondsof the peak; or no more than about 0.5 microseconds of the peak. Byshortly before the end of the decay of the luminescence may mean withinno more than about 50 microseconds of the end; or no more than about 40microseconds of the end; or no more than about 30 microseconds of theend; or no more than about 20 microseconds of the end; or no more thanabout 10 microseconds of the end. In some embodiments the end of thedecay may be taken as the time for the luminescence to decay to about20% of the peak value after a pulse; in some embodiments of theinvention, the end of the decay may be taken as the time for theluminescence to decay to about 15% of the peak value after a pulse; insome embodiments of the invention, the end of the decay may be taken asthe time for the luminescence to decay to about 10% of the peak valueafter a pulse; in some embodiments of the invention, the end of thedecay may be taken as the time for the luminescence to decay to about 5%of the peak value after a pulse; in some embodiments of the invention,the end of the decay may be taken as the time for the luminescence todecay to a value less than about 5% of the peak value after a pulse.

In implementation of some embodiments of the invention, the light sourceis pulsed for set periods of time. The light pulse is filtered in orderto more closely approximate a narrow band light source and is conductedto a sensor comprising a luminophor, for example along a fibre optic.The luminescence from the luminophor is then gathered and conducted backto the instrument where it is filtered to remove traces of the originallight source. The remaining light, that is the luminescence from theluminophor, is conducted back to a light sensing device where it isconverted to an electrical signal. The electrical signal is coarselycorrected for ambient light and amplified. The signal is then switchedto a number of accumulators. The timing of the light pulses andswitching to the various accumulators is controlled by timing logic. Theaccumulation time for each area is constant, but the areas can bedistributed after the light pulse completes as required. Thedistribution of integration areas is typically determined bymathematical modelling which includes the sensor lifetimecharacteristics and the measurement range required in order to maximiseaccuracy and minimise noise.

The sequence of light pulsing and accumulation is preferably repeatedduring an acquisition phase. Typically, there will be a “measurementpulse” followed by a “wait for completion” time, before an “interruptpulse” and “A to D conversion” time. This is implemented to allow thefinal light pulse and integration periods to complete before initiatingthe measurement.

In one embodiment of the invention, the majority of a cycle, for exampleat least about 80% or at least about 90%, is taken up by theaccumulators acquiring data. This is then followed by a short period,typically 5% of the cycle, when no pulses or integration take place.This allows the final accumulator value to be measured and recorded byan Analogue to Digital (A to D) converter and processor. There could bea software accumulator with A-D sampling at 10 MHz, for example. Finallya short reset phase, typically 5% of the cycle, allows the accumulatorsto be reset to a zero value. A typical cycle would be 1 second.

In a second embodiment of the invention the accumulator values aresampled at a fixed rate through the duration of the cycle. The linearslope through the samples acquired is proportional to the end value ofthe integration obtained in the first embodiment. The advantages of thismethod are:—

-   -   1) Samples immediately after the accumulator is reset can be        rejected from the calculation of the slope to reduce the effect        of the dielectric absorption of the capacitors used in the        accumulators;    -   2) Samples greater than a threshold, corresponding to the        saturation of the accumulator, can be rejected from the        calculation of slope, thus increasing the practical dynamic        range;    -   3) Oversampling and line fitting to the integral slope increases        the effective resolution of the A to D converter and reduces        quantisation noise;    -   4) Measurements are spread over the range of the A to D        converters, thus potentially reducing some of the effects due to        the inherent non-linearity of an A to D converter.

Thus, in a preferred embodiment of the invention, for each accumulatorthe accumulator value is sampled at intervals during the measurementsequence, and the values are processed to provide a slope which is usedin determination of the substance concentration.

Preferably, the periods of time for which respective accumulators areoperable are separated from each other by intervals in which none of theaccumulators is operable. Preferably for at least one pair of periods oftime for which the accumulators are operable, those periods areseparated from each other by an interval in which none of theaccumulators is operable, said interval being longer than the respectiveperiods of time for which any of the accumulators is operable. In suchan arrangement, the method may involve any of the optional featuresdiscussed above in connection with the method of the first aspect of theinvention.

In another embodiment of the invention, two measurement circuits arealternated between acquisition phases and reset phases. In practice thiscan be implemented by allowing the light and integration pulses tocontinue for the whole cycle, while holding alternate channels in reset.In such an arrangement the integral values may be calculated using theslope method described above This allows a higher sampling rate with theability to calculate the exponential lifetime a plurality of timesduring the measurement cycle rather than once per measurement cycle andwithout dropping data sampling.

Thus, in some embodiments of the invention, the data processing modulecomprises two channels, each of which comprises a plurality ofaccumulators which are controlled to be operable over a correspondingplurality of periods of time during decay of the luminescence, eachaccumulator accumulating values indicative of the intensity of theluminescence from the series of pulses in the measuring sequence, andwherein the measuring sequences are alternated between the two channels,the accumulators in one channel being reset after the end of themeasuring sequence using that channel, whilst a measuring sequence iscarried out using the other channel.

Thus, whilst the accumulators in one channel are being reset and waitingto become operable, the accumulators in the other channel are operable.In this manner the channels alternate.

In some embodiments of the invention, the periods of time for which therespective accumulators in a channel are operable are separated fromeach other by intervals in which none of the accumulators is operable.In some embodiments of the invention, for each accumulator theaccumulator value is sampled at intervals during the period for whichthe accumulator is operable, and the values are processed to provide aslope which is used in determination of the substance concentration.

In implementation of embodiments of aspects of the invention, whetherusing final accumulator values or slopes obtained from regular samplingof values from the accumulators, the data analysis may utilise amodified least squares method developed specifically for use inaccordance with the invention in order to calculate the exponentiallifetime.

The invention also extends to apparatus constructed and configured tocarry out the methods of the invention, including as options theoptional features of the invention as discussed above.

Thus, viewed from a further aspect, the invention provides apparatus formeasuring the concentration of a substance, comprising a sensorcomprising a luminescent material having a luminescent activity whosecharacteristics depend on the concentration of the substance, a lightsource which supplies a pulse of light to the sensor for a period oftime sufficient to cause luminescence of the luminescent material, theluminescence rising to a peak and then decaying after the end of thepulse of light, a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein the data processing module is configured so that in a measuringsequence there is a series of pulses of light spaced apart sufficientlyto permit the luminescence to decay between pulses, the signals whichare analysed to provide the data indicative of the concentration of thesubstance are those generated after each pulse of light has terminatedand the luminescence is decaying, and wherein the data processing modulecomprises a plurality of accumulators which are controlled to beoperable over a corresponding plurality of different periods of timeduring decay of the luminescence, each accumulator accumulating valuesindicative of the intensity of the luminescence from the series ofpulses in the measuring sequence.

The invention also extends to a software product for configuring a dataprocessing module of apparatus for measuring the concentration of asubstance, the apparatus comprising a sensor comprising a luminescentmaterial having a luminescent activity whose characteristics depend onthe concentration of the substance, a light source which supplies apulse of light to the sensor for a period of time sufficient to causeluminescence of the luminescent material, the luminescence rising to apeak and then decaying after the end of the pulse of light, and adetector for detecting the light emitted by the luminescence of theluminescent material and generating signals in response thereto, thedata processing module being for analysing the signals and for providingdata indicative of the concentration of the substance, wherein thesoftware product contains instructions which when run on the dataprocessing module will cause the data processing module to operate sothat in a measuring sequence there is a series of pulses of light spacedapart sufficiently to permit the luminescence to decay between pulses,the signals which are analysed to provide the data indicative of theconcentration of the substance are those generated after each pulse oflight has terminated and the luminescence is decaying, and wherein thedata processing module comprises a plurality of accumulators which arecontrolled to be operable over a corresponding plurality of differentperiods of time during decay of the luminescence, each accumulatoraccumulating values indicative of the intensity of the luminescence fromthe series of pulses in the measuring sequence.

The software product may be in the form of physical media carrying thesoftware, such as a CD, DVD, HDD, FDD or solid state memory module.Alternatively, the software product may be in the form of data providedfrom a remote location using suitable communications, such as over anetwork, for example the Internet.

Certain preferred embodiments of the invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a partial cross-section through a sensor probe forming part ofsensing apparatus embodying the invention;

FIG. 2 is schematic diagram of the sensing apparatus;

FIG. 3 is a schematic diagram of components within the sensingapparatus;

FIG. 4 is a circuit diagram of a measurement circuit component;

FIG. 5 is a circuit diagram of a sub-component of the measurementcircuit;

FIG. 6 is an approximate timing diagram relating to operation of thesensing apparatus;

FIG. 7 is a further approximate timing diagram relating to operation ofthe sensing apparatus;

FIG. 8 is a schematic diagram of components within a second sensingapparatus embodying the invention;

FIG. 9 is an approximate timing diagram relating to operation of thesecond sensing apparatus;

FIG. 10 is a graph relating partial pressure of oxygen to luminescentdecay; and

FIG. 11 is a table showing part of the implementation of a method inaccordance with the invention.

FIG. 1 shows a sensor probe 2 comprising an optical fibre core 4surrounded by a protective cladding layer 6. The optical fibre has atypical diameter of approximately 200 micrometres. At its tip it has aconvex-curved polymer layer 8 having embedded within it a number ofsilica-gel particles 10, of average diameter 5 micrometres. Theparticles 10 have an oxygen sensitive luminophor such asruthenium-chloride luminescent dye adsorbed on their surfaces.

FIG. 2 shows the sensor probe 2 connected via a port 18 to a base unit12. The base unit 12 contains a light-emitting unit 14 comprising alight-emitting diode (LED) and, for example, a 450 nm filter 14 a and alens 14 b. The filtering is preferably such that the light sourceapproximates a narrow-band light source. The light-emitting unit 14feeds into a beam splitter 16, to which the sensor probe 2 and areceiver unit 20 are optically-connected. The beam splitter 16 includesa dichroic filter 17 which reflects the light from the LED through theport 18 to the sensor probe 2, and then passes the luminescence lightfrom the sensor probe 2 to the receiver unit 20. The receiver unit 20,which may be an integrated component, comprises a coupling lens 22, along-pass filter 24 and a photo-detector 26 such as a photomultipliertube or avalanche photodiode, for example. The base unit 12 furthercontains a controller 28 which is electronically connected to thelight-emitting unit 14, by a connection 32, and to the photo-detector26, by a connection 30. The components of the base unit 12 may all bewithin a common housing or may be distributed.

In use, the controller 28 signals along the connection 32 to thelight-emitting unit 14 to cause an emission of light. This passesthrough the beam splitter 16 to the probe 2. On reaching the polymerlayer 8, the light causes the dye on the silica-gel particles 10 toluminesce. The luminescence response is dependent on the oxygenconcentration at the tip of the probe 2. Light from the luminescencetravels down the sensor probe 2 towards the base unit 12. It passesthrough the beam splitter 16 towards the receiver unit 20. Inside thereceiver unit 20, it passes through the coupling lens 22 and long-passfilter 24 before entering the photo-detector 26. The long-pass filter 24removes any stray light not due to the luminescence, which mighttypically peak at around 600 nm and, for example, the long-pass filter24 may be a 590 nm acetate filter. The electrical signal from thephoto-detector 26 passes along the connection 30 to the controller 28where it is received and processed as described below.

FIG. 3 figuratively shows the main components of the controller 28. Theconnection 30 from the photo-detector 26 is received in an analoguemeasurement circuit 34. The measurement circuit 34 feeds into ananalogue-to-digital (A to D) converter 36 which in turn feed into aprocessor 40. Timing and interface logic 38 connects to the measurementcircuit 34 by accumulator control lines 39A-39D and a reset control line41. The timing and interface logic 38 communicates with the processor 40by a data ready control line 43 and a timing state/configurationconnection 4; and has a connection 32 leading to the light-emitting unit14. A configuration memory 42, which may be volatile or non-volatile,having, for example, a magnetic, optical or silicon storage medium, isreadable by the processor 40 to determine configuration settings for theapparatus.

FIG. 4 shows the measurement circuit 34 in more detail. The connection30 from the photo-detector 26 passes through an amplifier 48 and is thensplit into four paths. Each path contains a high-speed electronic switch50A-50D, controlled by the timing and interface logic 38, before leadinginto a respective accumulator 52A-52D.

FIG. 5 shows a detailed view of one of the identical accumulators52A-52D. Each accumulator is a conventional operational-amplifiedaccumulator circuit, the output of which is the integral of the inputvoltage over time. It comprises an operational amplifier 56 with acapacitor 58 between its non-inverting input lead and its output lead,as well as an electronic reset switch 60 arranged to discharge thecapacitor 58 when closed. The reset switch 60 is controlled by thetiming and interface logic 38.

FIG. 6 shows the timing (not to scale) of the operation of thelight-emitting unit 14 and the accumulators 52A-52D. The timing andinterface logic 38, under the control of the processor 40, causes thelight-emitting unit 14 to emit an excitation pulse, which in thisembodiment has a duration of 2.0 μs. During this excitation pulse,luminescence from the silica-gel particles 10 rapidly increases. Afterthe pulse ends, the luminescence dies away exponentially, as shown inFIG. 6. The concentration of oxygen at the probe tip can be inferredfrom the rate of this exponential decay, which the present apparatus candetermine.

The signal from the photo-detector 26, which is a voltage correspondingto the light intensity, is coarsely corrected for ambient light and isamplified by the amplifier 48, before being switched, for optimallypositioned periods of time as described above, to one of the fouraccumulators 52A-52D by the operation of the switches 50A-50D, under thecontrol of the timing and interface logic 38. In one configuration, theamplified signal is switched to the first accumulator 52A during theperiod 0.5 to 2.5 μs after the end of the excitation pulse; it issubsequently switched to the second accumulator 52B during the period2.5 to 4.5 μs after the excitation pulse; then to the third accumulator52C during the period 36.75 to 38.75 μs after the pulse; and finally tothe fourth accumulator 52C during the period 145.5 to 147.5 μs after thepulse. These periods are indicated approximately in FIG. 6. The periodfor which each accumulator is active is thus 2.0 μs. The first twoperiods are consecutive, with no gap, and are where the rate of decay ofthe luminescence is relatively steep. The second two periods are spacedwidely from the second period and from each other, and are in a regionwhere the rate of decay is relatively shallow.

Other distributions of timing periods are, of course, possible.Typically, the distribution is determined by mathematical modellingwhich includes the lifetime characteristics of the sensor and themeasurement range required in order to maximise accuracy whileminimising noise. The configuration memory 42 can store severaldifferent configurations of timing regimes appropriate to the type ofsensor connected, for different situations, which are read by theprocessor 40 as needed. These may alternatively be stored in theprocessor itself; for example, in embodiments which do not have aseparate configuration memory.

Once the luminescence has substantially died away, another excitationpulse is emitted, and the accumulators 52A-52D are again switched onduring the same respective periods after each pulse. In the exemplaryconfiguration above, excitation pulses are emitted every 150microseconds.

This cycle of excitation pulses followed by switching of theaccumulators is repeated typically a few thousand times (e.g. over anacquisition phase of 0.9 seconds). The decay curve may typically remainsubstantially the same for each excitation pulse throughout theacquisition phase, but in any event the method of processing will givean average lifetime for the acquisition phase. The output of eachaccumulator 52A-52D after the acquisition phase is thus a sum ofintegrals at respective positions along the exponential decay curve.

FIG. 7 shows the operation of the timing and interface logic 38 duringtwo complete measurement cycles 62, each lasting about one second. Eachmeasurement cycle 62 consists of an acquisition phase 63 of about 0.9seconds, followed by a measurement phase consisting of a wait period 64in which the decay from the final light pulse of the acquisition periodis allowed to die away, followed by a read period 66, in which thevalues of the accumulators are read, and ending with a reset period 68of about 0.05 seconds, in which the capacitors 58 in the accumulators52A-52D are reset. The wait period 64 and the read period 66 total about0.05 seconds.

Although the various phases may be timed quite differently in otherembodiments, it is preferred in at least some embodiments that anacquisition phase lasts for about 90% of the total measurement cycle,with a subsequent read period of about 5% of the cycle, where no pulsesor accumulation takes place, and finally a reset period of about 5% ofthe cycle in which the accumulators are reset to a zero value.

In detail, at the start of operation, an initiate control line is takenlow, triggering the timing of all the other events. Immediately afterinitiation, a reset control line is taken low, causing the resetswitches 60 in the accumulators 52A-52D to open. This allows thecapacitors 58 in the accumulators 52A-52D to store charge and causes theaccumulators to start integrating their input signal voltages.Simultaneously with this, a light control trigger in the timing andinterface logic 38 causes the light-emitting unit 14 to be pulsed at apredetermined rate. During this acquisition phase 63, the output signalvoltages of the accumulators 52A-52D are the integral over time of theirinput signal voltages. A graph of a typical accumulator output signal isshown in FIG. 7. In practice, the output value rises in steps, as theoutput of the photo-detector is switched periodically to eachaccumulator; however, over many repetitions (typically thousands withina single acquisition phase), the output value can be considered to risesubstantially linearly as shown.

A measurement control line signals the end of the acquisition phase 63.The light-emitting unit ceases output and a wait period 64 commences.This allows the decay due to the final light pulse to be recorded by theaccumulators 52A-52D. The end of the wait period 64 and the start of theread period 66 is signalled by an interrupt signal on the interruptcontrol line 43 from the timing and interface logic 38 to the processor,causing the processor 40 to interrupt and read each accumulator 52A-52Dvalue in turn using the analogue-to-digital converter 36, and to storethe result for later processing. At the same time, the timing andinterface logic 38 uses the integration control lines 39A-39D to set allthe switches 50A-50D to the accumulators 52A-52D open, so that theaccumulators receive no more input signals. The accumulator outputsignals therefore remain steady and can be read by theanalogue-to-digital converter 36. The digitised values are readsequentially by the processor 40 for analysis as described below. Areset control line then signals the end of the read period 66 and thestart of a reset period 68 during which the switches 60 in theaccumulators 52A-52D are closed in order to discharge the capacitors 58in the accumulators, thereby resetting them.

After the accumulators 52A-52D have been reset, the measurement controlline drops low, taking the interrupt control and reset control with itand signalling the start of a second measurement cycle. The lightcontrol line causes the light-emitting unit 14 to start pulsing againand the process repeats.

In an alternative embodiment, instead of reading the accumulator 52A-52Doutput signal levels once per measurement cycle, after the end of theacquisition phase 63, the timing and interface logic 38 interrupts theprocessor 40 on several occasions throughout the measurement cyclecausing it to read the output of each accumulator on each occasion; thispreferably occurs at a fixed sampling rate, e.g. at around 200 Hz. Theprocessor stores each value along with a timestamp and a state valueindicating where in the measurement cycle the reading was taken (e.g.whether it is an initial value, a rising value, a final value, or avalue obtained during the reset period allocated for dielectricabsorption immediately after the end of the reset control line pulse).

For each accumulator, the processor 40 can compute an average of anyfinal-value readings, and these values can be used as the accumulatoroutputs in subsequent calculations.

Alternatively, the series of readings throughout the acquisition phasecan be used by the processor 40 to estimate the slope of thesubstantially-linear rise in output value; this may involve, forexample, performing a linear least-squares operation. Before thisoperation, however, any readings that are above a predeterminedsaturation value for the accumulator, or which are later in time thansuch a reading, can be discarded, thereby overcoming the problem offalse readings from any accumulator that has reached a point ofsaturation and consequent phase reversal (some op-amps that may be usedin the accumulators 52A-52D experience phase reversal when insaturation, such that the output of the op-amp can reduce below thesaturated value while still in saturation; the output is then actingnonlinearly and will give rise to erroneous readings if not eliminated).This gradient value can then be used to compute a theoretical finaloutput value by multiplying the gradient by the time between the end ofthe reset control signal and the start of the start of the interruptcontrol signal. The processor may also still compute an average of anyfinal-value readings, and compare this with the result.

If there is an inconsistency, for example because the accumulatorsaturated during the measurement cycle, the theoretical final value canbe used in place of the measured value. If an accumulator has not beenideally reset to zero, the theoretical final value should still becorrect, while the measured value will be in error. In tests, using acalculated theoretical final value instead of a measured value, hasproved to give good results notwithstanding the fact that theexponential luminescence lifetime might, in practice, vary slightly fromone light pulse to another.

The processor 40 may perform any of these calculations at any time, butin some arrangements it does so during the reset period when it need notbe sampling accumulator values.

FIG. 8 shows a third embodiment, similar to that of FIG. 3, but having afirst measurement circuit 134A and a second, identical measurementcircuit 134B. Each of these circuits is identical to the measurementcircuit 34 of the previous embodiments. Each measurement circuit has itsown accumulator control lines and reset control lines 141A, 141B. Thisembodiment has a total of eight accumulators, controlled as two blocksof four by the timing and interface logic 138. This allows a highersampling rate than the previous embodiments, since the two blocks can bereset independently of each other. There is an A to D converter 140 anda processor 142.

FIG. 9 illustrates the operation of this embodiment by showing twoadjacent measurement cycles. In contrast to the previous embodiments,the light-emitting unit 14 pulses constantly, and the measurement lineonly pulses briefly to signal the start of each new measurement cycle.At the start of a measurement cycle, the first measurement circuit 134Ais feed the output of the photo-detector 26 and switches this betweenits four accumulators. The output voltage of a typical accumulator 1 ofthis first measurement circuit is shown in FIG. 9; as before, it risessubstantially linearly as it receives luminescence from successive lightpulses. At the same time, a reset control line 2 for the secondmeasurement circuit 134B is held low, causing the capacitors in theaccumulators of the second circuit 134B to discharge. Part way throughthe measurement cycle, the reset control line 1 of the first circuit134A is taken high, causing its accumulators to reset, and at the sametime the reset control line 2 of the second circuit 134B is taken low,causing its accumulators to start integrating.

No wait and read periods are illustrated here, but they may be presentin practice. However, if the accumulator output values are sampledthroughout the acquisition phases, as explained above, it is possible todispense with a final read period altogether.

Because the accumulators of one or other of the measurement circuits134A, 134B are always active, this embodiment allows changes in assayconcentration to be determined faster than the typical one second.

In all of the embodiments, it is desirable to calibrate each accumulatorbefore using it, due to manufacturing tolerances between components ineach accumulator. The apparatus is therefore arranged so that theconnection 30 into the measurement circuit can, under the control of theprocessor 40, be fed by known, constant input signals, in place of theoutput of the photo-detector 26. The accumulator values recorded while acalibrated input is connected are then used to derive calibrationcoefficients to correct for any component tolerances.

Once theoretical or actual final values of each of the four accumulatorshave been determined for a measurement cycle, these are used by theprocessor 40 to derive an estimate of the lifetime of the exponentialdecay. The four values can be used to derive point values on the decaycurve by dividing by the number of samples and the width of theswitching period. The final estimate of the exponential decay lifetimeis arrived at by assuming that it lies in some range. A spread of valuesin the range is tested and the range is subsequently refined as a resultof the tested values. These steps are repeated until asufficiently-accurate lifetime estimate is arrived at. More detail ofthis process is given below.

For a given lifetime τ, the curve to be fitted is an exponential of theform

$\begin{matrix}{{f(t)} = {{A\; ^{\frac{- t}{\tau}}} + B}} & (1)\end{matrix}$

where A is a scaling factor and B is an offset.

A set of point values {(Y₁, t₁), (Y₂, t₂), (Y₃, t₃) . . . , (Y_(n),t_(n))} can be expressed as

$\begin{matrix}{Y_{i} = {{A\; ^{\frac{- t_{i}}{\tau}}} + B + ɛ_{i}}} & (2)\end{matrix}$

where ε_(i) is the magnitude of the error of the actual value from thecorresponding value on the curve.

The sum ε_(i) ² can be written as

$\begin{matrix}{{\sum\limits_{i = 1}^{n}ɛ_{i}^{2}} = {\sum\limits_{i = 1}^{n}\left( {Y_{i} - {A\; ^{\frac{- t_{i}}{\tau}}} - B} \right)^{2}}} & (3)\end{matrix}$

To fit the curve, the objective is to find values A and B that minimisethe least squares error term Σε².

Differentiating the error term with respect to A and then to B andlooking for stationary point gives

$\begin{matrix}{{\frac{\delta {\sum\limits_{i = 1}^{n}ɛ_{i}^{2}}}{\delta_{A}} = {{2{\sum\limits_{i = 1}^{n}\left\lbrack {\left( {Y_{i} - {A\; ^{\frac{- t_{i}}{\tau}}} - B} \right)\left( {- ^{\frac{- t_{i}}{\tau}}} \right)} \right\rbrack}} = 0}}{and}} & (4) \\{\frac{\delta {\sum\limits_{i = 1}^{n}ɛ_{i}^{2}}}{\delta \; B} = {{2{\sum\limits_{i = 1}^{n}\left( {Y_{i} - {A\; ^{\frac{- t_{i}}{\tau}}} - B} \right)}} = 0.}} & (5)\end{matrix}$

Simplifying gives

$\begin{matrix}{{{\sum\limits_{i = 1}^{n}Y_{i}} = {{A{\sum\limits_{i = 1}^{n}^{\frac{- t_{i}}{\tau}}}} + {nB}}}{and}} & (6) \\{{\sum\limits_{i = 1}^{n}{Y_{i}^{\frac{- t_{i}}{\tau}}}} = {{A{\sum\limits_{i = 1}^{n}^{\frac{{- 2}\; t_{i}}{\tau}}}} + {B{\sum\limits_{i = 1}^{n}{^{\frac{- t_{i}}{\tau}}.}}}}} & (7)\end{matrix}$

Solving for A and B then gives

$\begin{matrix}{{A = \frac{{n{\sum\limits_{i = 1}^{n}{Y_{i}^{\frac{- t_{i}}{\tau}}}}} - {\sum\limits_{i = 1}^{n}{Y_{i}{\sum\limits_{i = 1}^{n}^{\frac{- t_{i}}{\tau}}}}}}{{n{\sum\limits_{i = 1}^{n}^{\frac{{- 2}\; t_{i}}{\tau}}}} - \left( {\sum\limits_{i = 1}^{n}^{\frac{- t_{i}}{\tau}}} \right)^{2}}}{and}} & (8) \\{B = {\frac{{\sum\limits_{i = 1}^{n}{Y{\sum\limits_{i = 1}^{n}^{\frac{{- 2}\; t_{i}}{\tau}}}}} - {\sum\limits_{i = 1}^{n}{^{\frac{- t_{i}}{\tau}}{\sum\limits_{i = 1}^{n}{Y_{i}^{\frac{- t_{i}}{\tau}}}}}}}{{n{\sum\limits_{i = 1}^{n}^{\frac{{- 2}\; t_{i}}{\tau}}}} - \left( {\sum\limits_{i = 1}^{n}^{\frac{- t_{i}}{\tau}}} \right)^{2}}.}} & (9)\end{matrix}$

In this way, it is possible for the processor 40 to determine theparameters A and B that give a best fit to the measured data for anygiven estimate τ of the exponential decay. The quality of the fit can bedetermined from the magnitude of the sum of squares error term as givenin equation (3).

Although the present embodiments have shown four accumulators in ablock, it will be understood that any number of accumulators may beused. The value of n in the present equations can be changed to reflectthe number of accumulators without altering the validity of theequations.

A specific worked example of determining A and B now follows.

The objective is to minimise the sum of the squares of the errors, Σε²,for a given value of τ. Consider the case of four integrating windows,each of duration of 2 microseconds. The integrating windows are alwayspositioned at 0.5, 2.5, 36.75 and 145.5 microseconds after the end ofeach excitation pulse. The time between excitation pulses is 150microseconds.

The final accumulator values (either measured final accumulator valuesor calculated values using linear gradients) for each integration windowand their corresponding timings are:

t1 = 0.5 t2 = 2.5 t3 = 36.75 t4 = 145.5 (microseconds) Y1 = 7.5351 Y2 =7.2802 Y3 = 4.5942 Y4 = 1.8993 (Volts or Volts per seconds asapplicable)

Assume an estimate of lifetime τ=75 microseconds.

For simplicity of notation,

${{{Term}\mspace{14mu} 1}:={\sum\limits_{i = 1}^{n}^{\frac{- t_{i}}{\tau}}}},{{{Term}\mspace{14mu} 2}:={\sum\limits_{i = 1}^{n}^{\frac{{- 2}\; t_{i}}{\tau}}}},{{{Term}\mspace{14mu} 3}:={\sum\limits_{i = 1}^{n}{Y_{i}^{\frac{- t_{i}}{\tau}}}}}$and ${{Term}\mspace{14mu} 4}:={\sum\limits_{i = 1}^{n}{Y_{i}.}}$

Then

-   -   Term        1=e^((−0.5/75))+e^((−2.5/75))+e^((−36.75/75))+e^((−145.5/75))=2.716901951;    -   Term 2=e^((−1/75)+e)        ^((−5/75))+e^((−73.5/75))+e^((−291/75))=2.318224071;    -   Term        3=7.5351e^((−0.5/75))+7.2802e^((−2.5/75))+4.5924e^((−36.75/75))+1.8993e^((−145.5/75))=17.61402482;        and    -   Term 4=7.5351+7.2802+4.5924+1.8993=21.3088.

Introducing these into equations (8) and (9) gives

$\begin{matrix}{A = \frac{{n \times {Term}\; 3} - {{Term}\; 4 \times {Term}\; 1}}{{n \times {Term}\; 2} - \left( {{Term}\; 1} \right)^{2}}} \\{= \frac{\left( {4 \times 17.61402482} \right) - \left( {21.3088 \times 2.716901951} \right)}{\left( {4 \times 2.318224071} \right) - 2.716901951^{2}}} \\{= 6.641946192}\end{matrix}$ and $\begin{matrix}{B = \frac{{{Term}\; 4 \times {Term}\; 2} - {{Term}\; 1 \times {Term}\; 3}}{{n \times {Term}\; 2} - \left( {{Term}\; 1} \right)^{2}}} \\{= \frac{\left( {21.3088 \times 2.318224071} \right) - \left( {2.716901951 \times 17.61402482} \right)}{\left( {4 \times 2.318224071} \right) - 2.716901951^{2}}} \\{= {0.815820859.}}\end{matrix}$

Then the sum of squares of the errors,

$\begin{matrix}{{\Sigma \; ɛ^{2}} = {\left( {7.5351 - {A\; ^{({{- 0.5}/75})}} - B} \right)^{2} + \left( {7.2802 - {A\; ^{({{- 2.5}/75}}} - B} \right)^{2} +}} \\\left. {\left( {4.5924 - {A\; ^{({{- 36.75}/75})}} - B} \right)^{2} + 1.8993 - {A\; ^{({{- 145.5}/75})}} - B} \right)^{2} \\{= 0.117489577}\end{matrix}$

This is not the overall least squares fit to the data, but rather thebest least squares fit for the given value of lifetime, τ=75microseconds.

In order to find an accurate estimate of the decay lifetime, an initialassumption of the lifetime must be made. First, the maximum and minimumpossible acceptable lifetimes are defined. Once the likely range ofluminescence lifetime is known for a dye, and the light pulse andaccumulation periods and their relative positions have been optimisedfor the required measurement range, the upper and lower lifetime limitscan be chosen. In one example, a minimum lifetime, τ₀, is defined asbeing a tenth of the duration of an accumulation switching period, e.g.0.2 microseconds, and the maximum lifetime, τ₄, as being the timebetween successive light pulses, e.g. 150 microseconds.

The following algorithm is then performed, this being only one exampleof possible refinement algorithms:

1. Define five coefficients τ₀, τ₁, τ₂, τ₃, τ₄ 2. Set τ₀ := the minimumlifetime limit 3. Set τ₄ := the maximum lifetime limit 4. Set τ₂ :=(τ₀ + τ₄)/2 5. For a desired number of iterations do: a. Set τ₁ := (τ₀ +τ₂)/2 b. Set τ₃ := (τ₂ + τ₄)/2 c. For each of τ₀, τ₁, τ₂, τ₃, τ₄determine Σε² d. If τ₀ gives the smallest Σε² then τ₄ := τ₁, τ₂ := (τ₀ +τ₄)/2 e. If τ₁ gives the smallest Σε² then τ₄ := τ₂, τ₂ := τ₁ f. If τ₂gives the smallest Σε² then τ₀ := τ₁, τ₄ := τ₃ g. If τ₃ gives thesmallest Σε² then τ₀ := τ₂, τ₂ := τ₃ h. If τ₄ gives the smallest Σε²then τ₀ := τ₃, τ₂ := (τ₀ + τ₄)/2 6. Output τ₂ as the final estimate.

If τ₂ equals either of the minimum and maximum lifetime limits then theleast-squares minimum is outside the range defined by the minimum andmaximum limits.

The uncertainty in the final lifetime estimate is determined by thevalues of the minimum and maximum limits and the number of iterations,as follows:

uncertainty=(maximum limit−minimum limit)/2^((iterations−1)).  (10)

The number of iterations can therefore be determined to give a requiredlevel of uncertainty in the lifetime estimate.

The likelihood of a numeric overflow is limited since the primary causewould be the determination of the exponential, which in this algorithmis limited to the range of the minimum and maximum limits.

A specific worked example of applying this algorithm to arrive at aleast squares solution now follows. The numerical values are taken fromthe worked example above.

The integration pulse width effectively places a limit on the minimumlifetime which, in this case, shall be defined to be 0.05 microseconds.This is a fraction of the integration pulse width and gives the minimumlimit τ₀. Given that the time between pulses is 150 microseconds, it isreasonable to assume that the lifetime will be no greater than 150microseconds, and the maximum limit, τ₄, shall be defined to equal this.

From τ₀=0.05 μs and τ₄=150 μs, the algorithm gives τ₂=75.025 μs. τ₁ andτ₃ are then calculated as the mid-points according to the algorithm. Thefirst line in the table of FIG. 11 shows these values. Subsequent linesshow successive steps of the iterative process of refining the values ofτ₀-τ₄. The calculation of the exponential least-squares fit is asdescribed above. The lines marked SSQ show the sum of the squares of theerrors. The middle of the three SSQ values in the emphasised boxes isalways the minimum SSQ for that iteration; it is repeated in the columnto the right for clarity. Uncertainty is calculated as given above. Thearrows indicate how the exponential lifetimes, τ, and SSQ values aremoved from one iteration to the next iteration.

The final output value from this example is therefore a lifetime τ=54.84(two decimal places).

Once an estimate of the luminescence lifetime has been generated, theconcentration of oxygen can be determined using the Stern-Volmerequation

$\begin{matrix}{\frac{\tau_{zero}}{\tau} = {1 + {k_{SV}\lbrack Q\rbrack}}} & (11)\end{matrix}$

where τ is the lifetime for the measured concentration, τ_(zero) is thelifetime at zero concentration, τ_(SV) is the Stern-Volmer constant, andQ is the concentration of oxygen.

This equation can be improved by adding a Langmuir isotherm to betterreflect the actual data, giving

$\begin{matrix}{\frac{\tau_{zero}}{\tau} = {1 + {{k_{SV}\lbrack Q\rbrack}\alpha \frac{Kp}{1 + {Kp}}}}} & (12)\end{matrix}$

where K is the Langmuir constant, p is the partial pressure of oxygen,and α is an amalgamation of several constants.

Simplifying, a linear form relating the reciprocal of p to thereciprocal of (τ₀/τ−1) can be obtained thus:

$\begin{matrix}{\frac{1}{p} = {{D\frac{1}{\left( {\frac{\tau_{zero}}{\tau} - 1} \right)}} + C}} & (13)\end{matrix}$

where D and C in this equation are amalgamations of constant terms.

Where the deviation from the ideal Stern-Volmer equation is minimal, abroadly linear plot of the reciprocal of exponential lifetime againstconcentration is obtained. In order to allow for some deviation fromthis linear plot, a piecewise calibration scheme is employed.

FIG. 10 shows a typical plot of partial pressure of oxygen against thereciprocal of exponential lifetime for a ruthenium luminescent dye.

To calibrate the sensor, the following steps can be performed:

-   -   expose the sensor to a range of assay substance concentrations        over the desired range;    -   for each concentration, measure the luminescent lifetime; and    -   record each data pair of assay concentration and lifetime in the        configuration memory 42.

Where the performance of the luminophor is also dependent ontemperature, this approach can be modified thus:

-   -   expose the sensor to a range of assay substance concentrations        over the desired range of concentrations and temperatures;    -   for each concentration-temperature pair, measure the luminescent        lifetime; and    -   record each data triplet in the configuration memory 42.

Where a relationship can be found for extending calibration data toother temperatures without requiring further measurement, the data setsfor unmeasured temperatures can be created and entered into theconfiguration memory 42 as if the data points had been measuredempirically.

A simple method of sensor measurement, without temperature correction,is as follows:

-   -   extract the data from the configuration memory 42 and take the        reciprocal of the luminescent lifetimes;    -   measure the sensor lifetime in the unknown assay substance        concentration and take the reciprocal;    -   if the measured lifetime is outside the calibration data points,        then determine the two nearest data sets;    -   if the measured lifetime is within the calibration data points,        then determine the two calibration data sets either side of the        measured lifetime;    -   calculate the line through the calibration data sets; and    -   use the calculated line to convert the reciprocal of the        measured exponential lifetime to the measured concentration.

A more complex method of sensor measurement, without temperaturecorrection, is as follows:

-   -   extract the data from the configuration memory 42 and take the        reciprocal of the luminescent lifetimes;    -   measure the sensor lifetime in the unknown assay substance        concentration and take the reciprocal;    -   if the measured lifetime is outside the calibration data points,        then determine the two nearest data sets and calculate the line        through the calibration data sets;    -   if the measured lifetime is within the calibration data points,        then determine the two calibration data sets either side of the        measured lifetime and a third data set which is the nearest and        calculate the parabola through the three calibration data sets;        and    -   use the calculated line or parabola to convert the reciprocal of        the measured exponential lifetime to the measured concentration.

A simple method of sensor measurement with temperature correction is asfollows:

-   -   extract the data from the configuration memory 42 and take the        reciprocal of the luminescent lifetimes;    -   measure the sensor lifetime in the unknown assay substance        concentration and take the reciprocal;    -   determine the three nearest calibration data sets with the        proviso that all the points are not at the same temperature;    -   calculate the plane through the three calibration data sets; and    -   use the calculated plane to convert the reciprocal of the        measured exponential lifetime to the measured concentration.

In each of the embodiments above, the processor 40 has access to thenon-volatile configuration memory 42. This configuration memory maystore any of the following data:

-   -   sensor identification and configuration information;    -   the units of measurement for the assay substance;    -   the energy or time exposure of the sensor;    -   the allowed amount of energy or time exposure;    -   light pulsing and accumulator timings;    -   minimum and maximum acceptable luminescent lifetimes;    -   minimum and maximum acceptable sensor temperatures;    -   minimum and maximum acceptable assay concentrations; and    -   sensor dye and temperature calibration information.

Recording the allowed and actual energy or time exposure of the sensorallows the state of photo-bleaching to be monitored and the sensor to beflagged as due for replacement when required.

The apparatus may comprise sensors incorporating conventionaltemperature measuring devices such as thermocouples with band-gaptemperature sensors for cold junction correction. In such cases theconfiguration memory 42 may also store any of the following data:

-   -   Minimum and maximum un-calibrated cold junction temperatures;        Minimum and maximum calibrated cold junction temperatures; Cold        junction calibration information; Minimum and maximum        thermocouple voltages; Minimum and maximum calibrated        thermocouple temperatures; Thermocouple calibration information.

The use of the configuration memory 42 allows the instrument to beconfigured for a wide range of different situations. For example,sensors may be optimised for different measurement ranges, luminescentlifetimes and even different assay substances, all measured by the sameapparatus. Although the description above refers to measuring oxygenconcentration using a particular luminescent dye, it will be appreciatedthat properties and characteristics of substances other than oxygen mayadditionally or alternatively be measured by the same apparatus, perhapswith a different sensor probe attached, containing a different dye.

The invention may be viewed from a number of different aspects. Thus,viewed from a further aspect the invention provides a method ofmeasuring the concentration of a substance, using a sensor comprising aluminescent material having a luminescent activity whose characteristicsdepend on the concentration of the substance, a light source whichsupplies a pulse of light to the sensor for a period of time sufficientto cause luminescence of the luminescent material, the luminescencerising to a peak and then decaying after the end of the pulse of light,a detector for detecting the light emitted by the luminescence of theluminescent material and generating signals in response thereto, and adata processing module for analysing the signals and for providing dataindicative of the concentration of the substance; wherein in a measuringsequence there is a series of pulses of light spaced apart sufficientlyto permit the luminescence to decay between pulses, the signals whichare analysed to provide the data indicative of the concentration of thesubstance are those generated after each pulse of light has terminatedand the luminescence is decaying, and wherein the data processing modulecomprises a plurality of accumulators which are controlled to beoperable over a corresponding plurality of periods of time during decayof the luminescence, each accumulator accumulating a value indicative ofthe intensity of the luminescence after a pulse of light andaccumulating the values from the series of pulses in the measuringsequence; and wherein for each accumulator the accumulator value issampled at intervals during the measurement sequence, and the values areprocessed to provide a slope which is used in determination of thesubstance concentration.

Viewed from a further aspect the invention provides a method ofmeasuring the concentration of a substance, using a sensor comprising aluminescent material having a luminescent activity whose characteristicsdepend on the concentration of the substance, a light source whichsupplies a pulse of light to the sensor for a period of time sufficientto cause luminescence of the luminescent material, the luminescencerising to a peak and then decaying after the end of the pulse of light,a detector for detecting the light emitted by the luminescence of theluminescent material and generating signals in response thereto, and adata processing module for analysing the signals and for providing dataindicative of the concentration of the substance; wherein in a measuringsequence there is a series of pulses of light spaced apart sufficientlyto permit the luminescence to decay between pulses, the signals whichare analysed to provide the data indicative of the concentration of thesubstance are those generated after each pulse of light has terminatedand the luminescence is decaying, and wherein the data processing modulecomprises two channels, each of which comprises a plurality ofaccumulators which are controlled to be operable over a correspondingplurality of periods of time during decay of the luminescence, eachaccumulator accumulating a value indicative of the intensity of theluminescence after a pulse of light and accumulating the values from theseries of pulses in the measuring sequence; and wherein the measuringsequences are alternated between the two channels, the accumulators inone channel being reset after the end of the measuring sequence usingthat channel, whilst a measuring sequence is carried out using the otherchannel.

Viewed from a further aspect of the invention, there is provided methodof measuring the concentration of a substance, using a sensor comprisinga luminescent material having a luminescent activity whosecharacteristics depend on the concentration of the substance, a lightsource which supplies a pulse of light to the sensor for a period oftime sufficient to cause luminescence of the luminescent material, theluminescence rising to a peak and then decaying after the end of thepulse of light, a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein in a measuring sequence there is a series of pulses of lightspaced apart sufficiently to permit the luminescence to decay betweenpulses, the signals which are analysed to provide the data indicative ofthe concentration of the substance are those generated after each pulseof light has terminated and the luminescence is decaying, and whereinthe data processing module comprises a plurality of accumulators whichare controlled to be operable over a corresponding plurality of periodsof time during the decay of the luminescence, each accumulator detectinga value indicative of the intensity of the luminescence after a pulse oflight and accumulating the values from the series of pulses in themeasuring sequence.

Viewed from a further aspect, the invention provides apparatus formeasuring the concentration of a substance, comprising a sensorcomprising a luminescent material having a luminescent activity whosecharacteristics depend on the concentration of the substance, a lightsource which supplies a pulse of light to the sensor for a period oftime sufficient to cause luminescence of the luminescent material, theluminescence rising to a peak and then decaying after the end of thepulse of light, a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein the data processing module is configured so that in a measuringsequence there is a series of pulses of light spaced apart sufficientlyto permit the luminescence to decay between pulses, the signals whichare analysed to provide the data indicative of the concentration of thesubstance are those generated after each pulse of light has terminatedand the luminescence is decaying, and wherein the data processing modulecomprises a plurality of accumulators which are controlled to beoperable over a corresponding plurality of periods of time during decayof the luminescence, each accumulator accumulating a value indicative ofthe intensity of the luminescence after a pulse of light andaccumulating the values from the series of pulses in the measuringsequence.

Viewed from a further aspect, the invention provides apparatus formeasuring the concentration of a substance, comprising a sensorcomprising a luminescent material having a luminescent activity whosecharacteristics depend on the concentration of the substance, a lightsource which supplies a pulse of light to the sensor for a period oftime sufficient to cause luminescence of the luminescent material, theluminescence rising to a peak and then decaying after the end of thepulse of light, a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein the data processing module is configured so that in a measuringsequence there is a series of pulses of light spaced apart sufficientlyto permit the luminescence to decay between pulses, the signals whichare analysed to provide the data indicative of the concentration of thesubstance are those generated after each pulse of light has terminatedand the luminescence is decaying, and wherein the data processing modulecomprises a plurality of accumulators which are controlled to beoperable over a corresponding plurality of periods of time during decayof the luminescence, each accumulator accumulating a value indicative ofthe intensity of the luminescence after a pulse of light andaccumulating the values from the series of pulses in the measuringsequence; and wherein for each accumulator the accumulator value issampled at intervals during the measurement sequence, and the values areprocessed to provide a slope which is used in determination of thesubstance concentration.

Viewed from a further aspect, the invention provides apparatus formeasuring the concentration of a substance, comprising a sensorcomprising a luminescent material having a luminescent activity whosecharacteristics depend on the concentration of the substance, a lightsource which supplies a pulse of light to the sensor for a period oftime sufficient to cause luminescence of the luminescent material, theluminescence rising to a peak and then decaying after the end of thepulse of light, a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein the data processing module is configured so that in a measuringsequence there is a series of pulses of light spaced apart sufficientlyto permit the luminescence to decay between pulses, the signals whichare analysed to provide the data indicative of the concentration of thesubstance are those generated after each pulse of light has terminatedand the luminescence is decaying, and wherein the data processing modulecomprises two channels, each of which comprises a plurality ofaccumulators which are controlled to be operable over a correspondingplurality of periods of time during decay of the luminescence, eachaccumulator detecting a value indicative of the intensity of theluminescence after a pulse of light and accumulating the values from theseries of pulses in the measuring sequence; and wherein the measuringsequences are alternated between the two channels, the accumulators inone channel being reset after the end of the measuring sequence usingthat channel, whilst a measuring sequence is carried out using the otherchannel.

Viewed from a further aspect of the invention, there is provided asoftware product for configuring a data processing module of apparatusfor measuring the concentration of a substance, the apparatus comprisinga sensor comprising a luminescent material having a luminescent activitywhose characteristics depend on the concentration of the substance, alight source which supplies a pulse of light to the sensor for a periodof time sufficient to cause luminescence of the luminescent material,the luminescence rising to a peak and then decaying after the end of thepulse of light, and a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, the data processing module being for analysing thesignals and for providing data indicative of the concentration of thesubstance, wherein the software product contains instructions which whenrun on the data processing module will cause the data processing moduleto operate so that in a measuring sequence there is a series of pulsesof light spaced apart sufficiently to permit the luminescence to decaybetween pulses, the signals which are analysed to provide the dataindicative of the concentration of the substance are those generatedafter each pulse of light has terminated and the luminescence isdecaying, and wherein the data processing module comprises a pluralityof accumulators which are controlled to be operable over a correspondingplurality of periods of time during decay of the luminescence, eachaccumulator accumulating a value indicative of the intensity of theluminescence after a pulse of light and accumulating the values from theseries of pulses in the measuring sequence.

Viewed from another aspect of the invention, there is provided asoftware product for configuring a data processing module of apparatusfor measuring the concentration of a substance, the apparatus comprisinga sensor comprising a luminescent material having a luminescent activitywhose characteristics depend on the concentration of the substance, alight source which supplies a pulse of light to the sensor for a periodof time sufficient to cause luminescence of the luminescent material,the luminescence rising to a peak and then decaying after the end of thepulse of light, and a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, the data processing module being for analysing thesignals and for providing data indicative of the concentration of thesubstance, wherein the software product contains instructions which whenrun on the data processing module will cause the data processing moduleto operate so that in a measuring sequence there is a series of pulsesof light spaced apart sufficiently to permit the luminescence to decaybetween pulses, the signals which are analysed to provide the dataindicative of the concentration of the substance are those generatedafter each pulse of light has terminated and the luminescence isdecaying, and wherein the data processing module comprises a pluralityof accumulators which are controlled to be operable over a correspondingplurality of periods of time during decay of the luminescence, eachaccumulator accumulating a value indicative of the intensity of theluminescence after a pulse of light and accumulating the values from theseries of pulses in the measuring sequence; and wherein for eachaccumulator the accumulator value is sampled at intervals during themeasurement sequence, and the values are processed to provide a slopewhich is used in determination of the substance concentration.

Viewed from another aspect of the invention, there is provided asoftware product for configuring a data processing module of apparatusfor measuring the concentration of a substance, the apparatus comprisinga sensor comprising a luminescent material having a luminescent activitywhose characteristics depend on the concentration of the substance, alight source which supplies a pulse of light to the sensor for a periodof time sufficient to cause luminescence of the luminescent material,the luminescence rising to a peak and then decaying after the end of thepulse of light, and a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, the data processing module being for analysing thesignals and for providing data indicative of the concentration of thesubstance, wherein the software product contains instructions which whenrun on the data processing module will cause the data processing moduleto operate so that in a measuring sequence there is a series of pulsesof light spaced apart sufficiently to permit the luminescence to decaybetween pulses, the signals which are analysed to provide the dataindicative of the concentration of the substance are those generatedafter each pulse of light has terminated and the luminescence isdecaying, and wherein the data processing module comprises two channels,each of which comprises a plurality of accumulators which are controlledto be operable over a corresponding plurality of periods of time duringdecay of the luminescence, each accumulator accumulating a valueindicative of the intensity of the luminescence after a pulse of lightand accumulating the values from the series of pulses in the measuringsequence; and wherein the measuring sequences are alternated between thetwo channels, the accumulators in one channel being reset after the endof the measuring sequence using that channel, whilst a measuringsequence is carried out using the other channel.

The features and optional features of any of these additional aspects ofthe invention may be used together and may be used in conjunction withfeatures of the aspects of the invention discussed earlier.

The invention may also be used to measure the values of parameters otherthan concentration such as, for example, temperature or pH. Thus, viewedfrom another aspect the invention provides a method of measuring aparameter, using a sensor comprising a luminescent material having aluminescent activity whose characteristics depend on the value of theparameter, a light source which supplies a pulse of light to the sensorfor a period of time sufficient to cause luminescence of the luminescentmaterial, the luminescence rising to a peak and then decaying after theend of the pulse of light, a detector for detecting the luminescence andgenerating signals in response thereto, and a data processing module foranalysing the signals and for providing data indicative of the value ofthe parameter; wherein there is a measuring sequence comprised of aseries of pulses of light spaced apart sufficiently to permit theluminescence to decay between the pulses, the signals which are analysedto provide the data indicative of the concentration of the substance arethose generated after each pulse of light has terminated and theluminescence is decaying, and wherein the data processing modulecomprises a plurality of accumulators which are controlled to beoperable over a corresponding plurality of different periods of timeduring the decay of the luminescence, each accumulator accumulatingvalues indicative of the intensity of the luminescence from the seriesof pulses in the measuring sequence.

The other aspects of the invention may be modified mutatis mutandis soas to refer to measurement of a parameter in general.

1. A method of measuring the concentration of a substance, using asensor comprising a luminescent material having a luminescent activitywhose characteristics depend on the concentration of the substance, alight source which supplies a pulse of light to the sensor for a periodof time sufficient to cause luminescence of the luminescent material,the luminescence rising to a peak and then decaying after the end of thepulse of light, a detector for detecting the light emitted by theluminescence of the luminescent material and generating signals inresponse thereto, and a data processing module for analysing the signalsand for providing data indicative of the concentration of the substance;wherein in a measuring sequence there is a series of pulses of lightspaced apart sufficiently to permit the luminescence to decay betweenpulses, the signals which are analysed to provide the data indicative ofthe concentration of the substance are those generated after each pulseof light has terminated and the luminescence is decaying, and whereinthe data processing module comprises a plurality of accumulators whichare controlled to be operable over a corresponding plurality ofdifferent periods of time during the decay of the luminescence, eachaccumulator accumulating values indicative of the intensity of theluminescence from the series of pulses in the measuring sequence.
 2. Amethod as claimed in claim 1, wherein for each accumulator theaccumulator value is sampled at intervals during the measurementsequence, and the values are processed to provide a slope which is usedin determination of the substance concentration.
 3. A method as claimedin claim 1, wherein the data processing module comprises two channels,each of which comprises a plurality of said accumulators which arecontrolled to be operable over a corresponding plurality of differentperiods of time during decay of the luminescence, each accumulatoraccumulating values indicative of the intensity of the luminescence fromthe series of pulses in the measuring sequence; and wherein themeasuring sequences are alternated between the two channels, theaccumulators in one channel being reset after the end of the measuringsequence using that channel, whilst a measuring sequence is carried outusing the other channel.
 4. A method as claimed in claim 1, whereinthere are at least four accumulators, controlled to be operable over acorresponding number of different periods during the decay of theluminescence.
 5. A method as claimed in claim 3 wherein in each channelthere are at least four accumulators, controlled to be operable over acorresponding number of different periods during the decay of theluminescence.
 6. A method as claimed in claim 1, wherein there are firstand second accumulators which are timed to be active whilst theluminescence is decaying relatively steeply, the first accumulator beingactive over a first period of time and the second accumulator beingactive over an adjacent period of time which commences on, or arelatively short time after, the end of the first period of time.
 7. Amethod as claimed in claim 6, wherein there is a third accumulator whichis active over a third period of time, which commences a relatively longtime after the end of the second period of time, when there is asubstantially less steep rate of decay of the luminescence than duringthe first and second periods of time.
 8. A method as claimed in claim 7,wherein there is a fourth accumulator which is active over a fourthperiod of time, which commences a relatively long time after the end ofthe third period of time.
 9. A method as claimed in claim 1, whereineach accumulator is operable for a period which is less than 5microseconds during the decay of the luminescence.
 10. A method asclaimed in claim 1, wherein the periods of time over which therespective accumulators are operable are of substantially equalduration.
 11. A method as claimed in claim 1, wherein the periods overwhich the respective accumulators are active are determined byparameters stored in a configuration memory.
 12. A method as claimed inclaim 11, wherein the configuration parameters are stored in aconfiguration memory which is part of the sensor, and when the sensor isconnected to the control unit, a connection is made with theconfiguration memory, and the configuration parameters are read and usedby the data processing module.
 13. A method as claimed in claim 12,wherein the configuration memory which is part of the sensor stores, inaddition to the configuration parameters, the energy and/or timeexposure of the sensor.
 14. A method as claimed in claim 13, wherein theconfiguration memory which is part of the sensor stores, in addition,any or all of: the allowed amount of energy or time exposure; the dateof calibration of the sensor; the allowed period of time from the dateof calibration of the sensor; the date of first use of the sensor; theallowed period of time from the date of first use of the sensor; thenumber of times that the sensor has been used; the allowed number oftimes that the sensor may be used; sensor identification informationsensor configuration information; the units of measurement for the assaysubstance; light pulsing and accumulator timings; minimum and maximumacceptable luminescent lifetimes; minimum and maximum acceptable sensortemperatures; minimum and maximum acceptable assay concentrations; andsensor dye and temperature calibration information.
 15. A method asclaimed in claim 12, wherein the sensor also incorporate a temperaturemeasuring device.
 16. A method as claimed in claim 15, wherein theconfiguration memory also stores any or all of the following data:Minimum and maximum un-calibrated cold junction temperatures; Minimumand maximum calibrated cold junction temperatures; Cold junctioncalibration information; Minimum and maximum thermocouple voltages;Minimum and maximum calibrated thermocouple temperatures; Thermocouplecalibration information.
 17. Apparatus for measuring the concentrationof a substance, comprising a sensor comprising a luminescent materialhaving a luminescent activity whose characteristics depend on theconcentration of the substance, a light source which supplies a pulse oflight to the sensor for a period of time sufficient to causeluminescence of the luminescent material, the luminescence rising to apeak and then decaying after the end of the pulse of light, a detectorfor detecting the light emitted by the luminescence of the luminescentmaterial and generating signals in response thereto, and a dataprocessing module for analysing the signals and for providing dataindicative of the concentration of the substance; wherein the dataprocessing module is configured so that in a measuring sequence there isa series of pulses of light spaced apart sufficiently to permit theluminescence to decay between pulses, the signals which are analysed toprovide the data indicative of the concentration of the substance arethose generated after each pulse of light has terminated and theluminescence is decaying, and wherein the data processing modulecomprises a plurality of accumulators which are controlled to beoperable over a corresponding plurality of different periods of timeduring decay of the luminescence, each accumulator accumulating valuesindicative of the intensity of the luminescence from the series ofpulses in the measuring sequence.
 18. A non-transitory computer-readablemedium having stored thereon instructions for configuring a dataprocessing module of apparatus for measuring the concentration of asubstance, the apparatus comprising a sensor comprising a luminescentmaterial having a luminescent activity whose characteristics depend onthe concentration of the substance, a light source which supplies apulse of light to the sensor for a period of time sufficient to causeluminescence of the luminescent material, the luminescence rising to apeak and then decaying after the end of the pulse of light, and adetector for detecting the light emitted by the luminescence of theluminescent material and generating signals in response thereto, thedata processing module being for analysing the signals and for providingdata indicative of the concentration of the substance; wherein thenon-transitory computer-readable medium contains instructions which whenrun on the data processing module will cause the data processing moduleto operate so that in a measuring sequence there is a series of pulsesof light spaced apart sufficiently to permit the luminescence to decaybetween pulses, the signals which are analysed to provide the dataindicative of the concentration of the substance are those generatedafter each pulse of light has terminated and the luminescence isdecaying, and wherein the data processing module comprises a pluralityof accumulators which are controlled to be operable over a correspondingplurality of different periods of time during decay of the luminescence,each accumulator accumulating values indicative of the intensity of theluminescence from the series of pulses in the measuring sequence. 19-20.(canceled)
 21. A method as claimed in claim 2, wherein each accumulatoraccumulates a value indicative of the intensity of the luminescenceafter a pulse of light and accumulates the values from the series ofpulses in the measuring sequence.
 22. A method as claimed in claim 3,wherein each accumulator accumulates a value indicative of the intensityof the luminescence after a pulse of light and accumulates the valuesfrom the series of pulses in the measuring sequence.